In the fabrication process of integrated circuit chips, certain tests, for example, know good die (KGD), know good set (KGS) and know good module (KGM) tests, need to be conducted on a particular die. Most of the conventional methods require the use of a die-attach paddle and thus a permanent assembly of a heat sink to the die-attach paddle. The die-attach paddle must be precisely formed by a photolithography process and an etching process. The die-attach paddle also presents an additional layer of heat insulation which impedes the thermal conductance between the IC chip and the heat sink. Moreover, all the conventional methods require a permanent attachment of a heat sink to a leadframe which makes the testing of an IC chip impossible after it is attached to the heat sink. The capability of testing an IC chip without permanently attaching the chip to a leadframe is very desirable in the fabrication process of IC devices.
In the conventional IC chip fabrication technology, the chips are die-sawed from a wafer one at a time. These chips are called bare chips since they have not been encapsulated (or protected) in plastic packages. The bare chips can be easily damaged by dust, moisture, radiation and temperature variations in the environment. The bare chips can also be damaged during handling due to their extremely small size. To avoid damages to the circuits on the chips, the bare chips can not be stacked during handling and must be picked-up with extreme care. The performance of a KGD test on the bare chips is therefore almost impossible. It is desirable to provide a means for moving the chips without incurring damages during the testing process.
In the conventional fabrication process, a layer of an adhesive tape is first attached to the back of a wafer before the die-sawing process. The bare chips are still attached to the adhesive tape after the die-sawing process. Any potential damages to the chips by falling off the wafer after separation can thus be avoided. After the separation, the bare chips are transferred to the die-attach paddles on leadframes by vacuum means. Any necessary test on the bare chips must therefore be conducted when the chips are still attached to the wafer (a so-called wafer level test) before the separation. The questionable chips are marked by ink and then separated from the wafer. The drawback of this testing method is that there is no guarantee that the bare chips are still good after separation by die-sawing. It is therefore advantageous to provide a method of testing a bare chip immediately after its separation from a wafer and its transfer to a die-attach paddle. Of course, this would require that a die-attach paddle to be independent from a leadframe.
It is therefore an object of the present invention to provide a method and apparatus for detachably mounting a heat sink to a leadframe such that the heat sink, after having an IC die attached, can be detached from the leadframe for testing of the die.
It is a further object of the present invention to provide a method and apparatus for detachably mounting a heat sink to a leadframe and then mounting an IC die to the heat sink such that the IC die and the heat sink can be detached from the leadframe for testing.